Achromats are optical elements consisting of multiple lenses made from different materials to compensate for the chromatic aberration. Typically an achromatic doublet is produced from a crown glass and flint glass. Dependent on the type, the lenses can either be cemented with special optical glue or air-spaced using mechanic mounting. An achromatic doublet grants correction of focal lengths for two wavelengths (typically red and blue light) and significantly reduces the overall chromatic aberration. The achromats can be made in various specifications with both convex and concave surfaces allowing for positive and negative focal lengths. More freedom to specify the parameters allows also for correction of image quality: decrease of spherical aberration, elimination of coma and precise focal point for  a single wavelength (diffraction limit).